Synthesis, Characterization, and Evaluation of Telechelic Acrylate

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Synthesis, Characterization, and Evaluation of Telechelic Acrylate Oligomers and Related Toughened Epoxy Networks Ajit K. Banthia, Prakash N. Chaturvedi, Vandana Jha, and Veera N. S. Pendyala Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, India

Telechelic elastomeric carboxyl end-capped acrylates have been synthesized and thoroughly characterized. The effect of initiator [4,4'azobis(4-cyanovaleric acid)] and chain-transfer agent (dithiodiglycolic acid) concentration, polymerization technique and temperature, and nature of solvent(s) on the gelation, molecular weight, and carboxyl functionality of the resulting oligomer has been explored in detail. Acrylate oligomers (M ≤15,000)exhibited extremely good miscibility with the conventional epoxy resin (diglycidyl ether of bisphenol A), but precipitated as a distinct (single-mixed) phase during network formation. Incorporation of the reactive elastomer (4-10%) significantly enhanced the impact strength of the epoxy network and correlated well with energy absorption of the relaxation process. Carboxyl-terminated telechelic ethylhexyl acrylate oligomers were found to be a potential elastomeric toughening agent for epoxy resin. n

TOUGHNESS IS THE ABILITY OF A MATERIAL TO ABSORB ENERGY and undergo large permanent set without rupture. For many engineering ap plications, toughness is often the deciding factor. Plastics, because of their inherent brittleness, are an important candidate for toughening studies, with special emphasis on the brittleness of cross-linked glassy polymers. Epoxy

0065-2393/89/0222-0343$06.00/0 © 1989 American Chemical Society

Riew; Rubber-Toughened Plastics Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

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R U B B E R - T O U G H E N E D PLASTICS

resin (I), a versatile glassy network, exhibits excellent resistance to corrosion and solvents, good adhesion, reasonably high glass-transition temperatures (T ), and adequate electrical properties. However, its poor fracture tough ness has been the subject of intense investigations throughout the world.

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g

Approximately 5-20% of a low-molecular-weight reactive liquid elas tomeric oligomer, initially miscible and homogeneously dispersed in the host resin, may provide a multiphase toughened network (2) on curing. To optimize toughening, parameters such as modifier structure, molec ular weight, solubility, and elastomer concentration must be clearly de fined. These parameters are responsible for the dynamics of multiphase morphology. For effective toughening of the epoxy resin, the dispersed elastomeric phase must be grafted, at least to a certain extent, to the matrix resin (3-5). However, excessive grafting of the elastomeric phase may lead to the for mation of a single-phase morphological system (6), with disastrous results. These factors suggest the use of various reactive oligomers, such as carboxyl-amine end-capped acrylonitrile-butadiene oligomer (7-10), carboxylterminated polyisobutylene (11), and functionalized siloxane oligomers (12-14). Some commercially important oligomers used for toughening have a built-in unsaturation in the backbone that enhances their thermal and oxidative degradation. Acrylate-based reactive oligomers may be an ideal choice for the toughening of epoxy resin because of its comparatively better oxidative and thermal stability and its complete miscibility with diglycidyl ether of bisphenol A ( D G E B A ) . Simultaneous interpenetrating network (SIN) has been used to improve the impact strength of epoxy resin by simultaneously polymerizing it with η-butyl acrylate (15). This chapter deals with synthesis, characterization, and evaluation of 2-ethylhexyl acrylate (EHA) based carboxyl-terminated oligomers as a toughening agent for DGEBA.

Experimental Details Materials. T h e monomers ethyl acrylate (EA), butyl acrylate (BA), and E H A were obtained commercially. They were purified by washing twice with aqueous sodium hydroxide solution (10% w/v) to remove inhibitor and then twice with distilled water. They were then dried over anhydrous calcium chloride for 48 h and distilled under reduced pressure. 4,4'-Azobis(4-cyanovaleric acid) (ABCVA) (1) and azobisisobutyronitrile recrystallized from ethanol were used as free-radical initiators and stored in the dark at - 2 5 °C. D G E B A liquid epoxy resin (Dobeckot 520F, epoxy equivalent weight 190 g) was used. T h e amine curing agent, bis(4-aminocyclohexyl)methane (PACM-20), was used as supplied. Synthesis of Carboxyl-Terminated Acrylate Oligomers. The following is a typ ical synthesis of carboxyl-terminated telechelic E H A oligomer by bulk polymeriza tion. Approximately 20 g of E H A monomer was placed in a 100-mL two-necked glass


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Telechelic Acrylate Oligomers and Epoxy Networks

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reactor fitted with a stirrer, thermometer, and gas inlet. The requisite amount of A B C VA was added. After the system was well purged with an inert gas, the reaction was rapidly brought to the desired temperature and allowed to continue for 10-30 min. The reaction mixture was diluted with a suitable solvent such as aromatic hydrocarbons and immediately quenched to room temperature. Unreacted A B C V A was precipitated overnight and subsequently filtered. Unreacted monomer, if any, and the solvent were removed under vacuum on a rotary evaporator until a constant weight was obtained. A typical preparation by a solution polymerization technique uses inhibitor-free monomer (0.1 mol), 2-propanol (3.0 mol) and A B C VA (0.015 mol). The materials were introduced into a flask, and nitrogen was slowly bubbled through them for 15 min to remove dissolved oxygen. T h e mixture was heated and stirred at the desired temperature for — 12 h. The solution was cooled, filtered, and evaporated to dryness. Unreacted A B C V A was removed as previously described. Isothermal free-radical bulk polymerization of acrylic acid esters was investigated at 100 °C with a differential scanning calorimeter (Perkin-Elmer DSC-2). A B C V A was added to initiate the polymerization. The enthalpies of the reaction and overall rate constants were calculated from the areas between the D S C curves and the baseline, which was obtained by back-extrapolation of the straight line recorded after the completion of polymerization. The D S C curves were calibrated with the melting enthalpy of indium. The number-average molecular weight (M ) of the purified oligomers was de termined in chloroform at 37 °C by using a vapor pressure osmometer (Knauer). Carboxyl functionality of the oligomers was determined by potentiometric acid-base titration in nonaqueous medium, with alcoholic K O H as titrant (14). In dicators were also utilized to complement the potentiometric studies. T h e epoxy groups were titrated according to the procedure discussed in the literature (14, 16). The solubility parameters of the oligomers and D G E B A were determined by the recently developed method of Banthia et al. (17). n

Epoxy Network Formation and Evaluation.

One-Stage Method. T h e epoxy

resin, the elastomer mixture (with 1% triphenylphosphine), and the curing agent were heated in separate beakers under vacuum (5 mm Hg) at 60 °C for 15 min to remove air bubbles. T h e n they were mixed, poured into hot aluminum molds, and cured at 100 °C for 2 h, followed by 160 °C for 2.5 h. Two-Stage Method. The epoxy resin and the elastomer mixture were prereacted in the presence of 1% triphenylphosphine at 100 °C for 2 h. The prereacted epoxy-elastomer mixture and the curing agent were heated in separate beakers under vacuum (5 mm Hg) at 60 °C for 15 min to remove air bubbles. T h e n they were mixed, poured into hot aluminum molds, and cured at 160 °C for 2.5 h. The glass-transition temperatures of the epoxy network were measured with a differential scanning calorimeter (Perkin-Elmer D S C - 2 ) . Dynamic mechanical spec troscopy was studied by using a dynamic mechanical thermal analyzer (Polymer Laboratories, P L - D M T A ) in the temperature range -120 to 165 °C at 100 H z , with a heating rate of 3 °C per min. Impact properties of some selected samples were determined with an instrumented falling-weight impact tester.

Results and Discussion Synthesis a n d C h a r a c t e r i z a t i o n of T e l e c h e l i c A c r y l a t e O l i g o mers. For higher acrylate monomers, termination of free-radical poly-


346



merization in bulk occurs predominantly via combination mode rather than diffusion-controlled termination. Furthermore, the onset of the gel effect is presumed to be delayed considerably as alkyl group length increases. Is othermal bulk polymerization of acrylate oligomers of interest provides a clue to the time of completion of free-radical polymerization reactions and to whether gelation took place in our systems at any stage of polymer or oligomer formation. The course of the isothermal bulk polymerization of Ε A, ΒA, and E H A at 100 °C is presented in Figure 1. The polymerization trends of these acrylates are similar. After the initial brief period, a gradual increase in the exotherm is observed and a well-expressed gel effect sets in. However, with increasing length of the alkyl groups in the acrylic acid ester, the intensity of the gel effect decreases and the conversion at the onset of the gel effect increases. In the polymerization of E H A , the gel effect is not observed at any stage of polymer and oligomer formation. Within the polymerization temperature range of our interest, 100% conversion takes place within 10-20 min. Similar observations of isothermal bulk polymerization of methacrylate have been reported by other workers (IS). The same group reported that the gel effect in the polymerization of methyl acrylate starts immediately after the beginning of the reaction. The shift of the onset of the gel effect to higher conversion and its ultimate suppression cannot be explained by the theory of diffusion-controlled ter mination in the highly viscous media, as a consequence of higher monomer viscosity. However, the shift can be explained by considering the fact that in the present system the increasing length of the alkyl group enhances the mobility of the polymer chains because of enhanced shielding of the carbonyl groups by the segmental rearrangement of these alkyl groups (18-22). Low-

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Time (seconds) Figure 1. Isothermal bulk polymerization of EA, BA, and EHA at 100 °C in the presence of 1.0 x 10 mollL ABCVA. 2


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Telechelic Acrylate Oligomers and Epoxy Networks 347

ering of the glass-transition temperatures of polyacrylates (Table I) can also be explained in the same way. The enthalpy of polymerization of these acrylates decreases with the length of the alkyl group (Table II). The polymerization enthalpies for the same monomers at different temperatures (80-120 °C) are within the limits


of experimental error, estimated to be ± 2 % . The molecular weight of E H A oligomers decreases with increasing tem perature (Table III). The molecular weight decreases dramatically with the increasing polymerization temperature up to 100 °C in bulk (Table III). As expected, the increase in the initiator concentration resulted in a decrease of the molecular weight. However, the effect of initiator concentration is relatively small, as compared to the effect of polymerization temperature on the molecular weight.

Table I. Glass-Transition Temperatures of Some Representative Elastomeric Acrylates [CH -CH(COOR)] 2

B

R

T , °C g

-25 -55 -60 -57 -45 -55

C2H5

n-C H 4

9

C5H11

CeH 13 C H r~ CH(C 2 H 5)—C 4 H

9

Table II. Polymerization Enthalpies of E H A at Different Temperatures ΔΗρ, kj/mol

Temperature (°C)

77.0 77.5 78.0 79.5

80 90 100 120

Table HI. Number-Average Molecular Weight and the Functionality (f) of Carboxyl-Terminated E H A Oligomers Prepared by Bulk Polymerization Sample No. 1 2 3 4 5 6 7

EHA, β 20.0 20.0 20.0 20.0 20.0 20.0 20.0

ABCVA, g 2.0 2.0 2.0 2.0 2.5 3.0 4.0

Temperature, °C 120 100 90 80 100 100 100

M

n

10,231 11,564 12,325 15,026 10,078 10,196 9,525


f 2.07 2.01 2.02 2.04 1.99 2.01 2.03


348


Even these high-molecular-weight E H A oligomers were miscible with the conventional epoxy resin, D G E B A . This observation was supported by our studies pertaining to the determination of solubility parameters of poly mers (17). The experimental solubility parameters of E A , B A , E H A oligo mers, and D G E B A (Table IV) increase with increasing length of the alkyl group. This increase can be correlated with the segmental rearrangement and its effect on carboxyl group shielding. Presumably, the polymer-solvent interaction is enhanced by these factors. The close proximity of the solubility parameters of E H A oligomers ( δ is 9.60 c a l c m ) and D G E B A resins ( δ is 9.73 c a l c m ) ensures their complete miscibility up to — 20 wt % of the E H A oligomers in epoxy resin. The measured functionality of carboxyl-terminated ethylhexyl acrylate ( C T E H A ) oligomers prepared by bulk polymerization technique was in the range of 1.99-2.07 (Table III). This characteristic carboxyl functionality value of C T E H A oligomer was found to be independent of the changes in the polymerization temperatures and initiator concentrations. Thus, these C T E H A polymers-oligomers are essentially telechelic in nature. 1/2

τ

1/2

3/2

τ

3/2

Dilute-Solution Polymerization of Acrylates. Role of Disulfide Compounds as Chain-Transfer Agents. Solution polymerization of acrylic acid esters is better predicted and controlled, and dissipation of heat is easier than with bulk polymerization. Lack of heat dissipation could promote the onset of gelation for most of the lower acrylates. Several workers have studied the use of various symmetrical difunctional compounds (e.g., aromatic and aliphatic disulfide compounds) for the synthesis of telechelic polymers and oligomers by the free-radical polymerization technique (23-26). Dithiodiglycolic acid ( D T D G A ) (2) was used as a chain-transfer agent. It can evidently react with the growing polymer radicals through cleavage of the sulfur-sulfur bond to provide a carboxyl end group. A representation of the tentative reaction mechanism is shown in Scheme I. This reaction will reduce the probability of chain termination of growing acrylate radicals by combination or disproportionation. In turn, disproportionation will be

Table IV. Solubility Parameters of Some Potential Qligomeric Acrylates and D G E B A

Oligomers

Theoretical

Experimental

8.52 8.96 9.32 9.52 9.71

8.61 9.00 9.30 9.60 9.73

0

Methyl acrylate Ethyl acrylate Butyl acrylate Ethylhexyl acrylate DGEBA

13

NOTE: All results for δ are given as the square root of calories per cubic centimeter at 25 ° C . "Small's method. Our results. τ

fo


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BANTHIA E T AL.

Telechelic Acrylate Oligomers and Epoxy Networks 349

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Β A > E H A . F u r thermore, nearly 100% conversion of acrylate monomers can be achieved. The molecular weight of these acrylate oligomers can easily be controlled by polymerization temperature, initiator concentration, and chain-transfer agent concentration. Increasing initiator or chain-transfer agent concentra tion, as well as polymerization temperature, decreases the molecular weight. Carboxyl-terminated E H A oligomers (8 9.60 c a l T

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/cm

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, M

n

< 15,000)

were completely miscible with D G E B A ( δ 9.73 c a l / c m ) at room tem τ

1/2

3/2

perature. Severe chain transfer to solvents appeared to decrease the overall car boxyl functionality of the resulting acrylate oligomer in dilute-solution freeradical polymerization. The presence of two distinct T and β-relaxation temperatures in all the g

cases under investigation indicates the multiphase nature of these systems.


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Telechelic Acrylate Oligomers and Epoxy Networks

357


There is a distinct shift in the β-relaxation temperature as the E H A concentration increases beyond 10 wt %. Furthermore, the formation of an epoxy-rubber reactive adduct may be the prime reason for the formation of a broad region of damping area covering both initial plastics-elastomeric phases. This damping area may be an important governing factor in the enhancement of toughening and may have synergistic effects. Similar effects have been observed in various other toughened systems. Preliminary results indicated that impact strength of the C T E H A mod ifier epoxy network increases with increasing modifier concentration (210%). However, nonreactive E H A oligomer failed to enhance the toughening characteristic of the epoxy network. Carboxyl-terminated telechelic ethylhexyl acrylate oligomers might be a potential elastomeric toughening agent.

Acknowledgments A. K. Banthia expresses his appreciation to the Department of Science and Technology, Government of India, for the financial support needed to carry out the present investigation. The bis(4-aminocyclohexyl)methane sample was donated by Ε. I. du Pont de Nemours and Company.

References 1. Lee, H.; Neville, K. Handbook of Epoxy Resins; McGraw-Hill: New York, 1947. 2. Bucknall, C. B. Toughened Plastics;Applied Science: New York,

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3. McGarry, F. J.; Willner, A . W. Toughening of an Epoxy Resin by an Elastomeric Second Phase; R 68-8, Massachusetts Institute of Technology: Cambridge, March 1968. 4. Bucknall, C . B . ; Yoshii, T. Br. Polym. J. 1978, 10, 53. 5. Scarits, P. R . Sperling, L. H. Polym. Eng. Sci. 1979, 19, ;

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6. Noshay, Α.; Robeson, L . H. J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 689. 7. Rowe, E. H.; Siebert, A . R.; Drake, R. S. Mod.

Plast. 1970, 49,

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8. Riew, C . K . ; Rowe, E. H.; Siebert, A . R. In Toughness and Brittleness of Plastics; Deanin, R.; Crugnola, A. M., E d s . ; Advances in Chemistry 154; Chemical Society: Washington, D C , 1976; p 326.

American

9. Riew, C . K. Rubber Chem. Technol. 1981, 54, 374. 10. Drake, R. K.; Egan, D . R.; Murphy, W. T. Org. Coat. Appl. Polym. Sci. Proc. 1982, 46, 392. Slysh, R. In Epoxy Resins; Advances in Chemistry 92; American Chemical So ciety: Washington, D C , 1970; p 108. 12. Banthia, A . K.; Riffle, J. S.; Steckle, W. P.; Yilgor, I.; Wilkes, G.; M c G r a t h , J.

11.

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E. In Proceedings of the International Conference on Structure—Property Re lationship of Rubber; Bhowmick, A . K . ; D e , S. Κ., E d s . ; Indian Institute of Technology: Kharagpur, India, 1980; P-217. 13.

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In Epoxy Resin Chemistry II; Bauer, R. S., E d . ; ACS Symposium Series 221; American Chemical Society: Washington, D C , 1983; p 21. Sperling, L . H.; Friedman, D. W. J. Polym. S c i , Part A-2 1969, 7, 425. Standard Test Method for Epoxy Content Epoxy Resins, ASTM Designation: D 1652-73. Pendyala, V. N. S.; Shareef, Κ. Μ. Α.; Banthia, A. K., unpublished results. Malavasec, T.; Osredkar, U . ; Anzur, L . ; Vizovisek, I. J. Macromol. Sci. Chem. 1986, A23, 853. Cardenas, J. N.; O'Driscoll, K. F. J. Polym. Sci., Polym. Chem. Ed. 1976, 14, 883; 1977, 15, 2097. North, A. M.; Reed, G. A. J. Polym. Sci., Part A 1963, 1, 1311. Benson, S. W.; North, A. M . J. Am. Chem. Soc. 1962, 84, 935. Burkhart, R. D. J. Polym. Sci., Part A 1965, 3, 883. Pierson, R. M.; Costanza, A. J.; Weinstein, A. H . J. Polym. Sci. 1955, 17, 221, 319. Athey, R. D.; Mosher, W. A.; Weston, N. W. J. Polym. Sci. 1977, 15, 1423. Athey, R. D.; Mosher, W. A.; Weston, N. W. Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 1979, 20, 20. Athey, R. D. Prog. Org. Coat. 1979, 7, 289. Yilgor, I.; Yilgor, E.; Banthia, A. K.; Wilkes, G. L.; McGrath, J. E . Polym. Bull. 1981, 4, 323. Pircivanoglu, V. S.; Ward, T.; Yilgor, I.; Banthia, A. K.; McGrath, J. E . Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 1981, 22, 174.

RECEIVED for review February 11, 1988. 27, 1988.

ACCEPTED

revised manuscript December


Synthesis, Characterization, and Evaluation of Telechelic Acrylate

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